1. Field of the Invention
The present invention relates to a path fault recovery method, a switching-back method after recovery from a fault, and a node using the same, and particularly concerns a fault recovery method for an optical path using a signaling message in an optical network and a switching-back method for an optical path after recovery from a fault.
2. Description of the Prior Art
As to an optical network constituted by an OXC (Optical Cross-connect) and an ADM (Add/Drop Multiplexer) or a SONET/SDH XC (Synchronous Optical Network/Synchronous Digital Hierarchy Cross-connect), a conventional example of a fault recovery method in the event of a fault on an active path is described in “draft-ietf-ccamp-gmpls-architecture-01.txt” (hereinafter, referred to as Document 1) which is an Internet draft of IETF (Internet Engineering Task Force). The fault recovery method for switching an active path and a standby path in Document 1 is further classified as “1+1” method, “1:1” method, and “Shared” method.
The “1+1” method sets a standby path having a path not overlapping with an active path when the active path is set. In this method, data from a client is supplied to both of the active path and the standby path on the transmitting ends of the paths and only data from the active path is received on the receiving ends. Moreover, in this method, when the active path has a fault, switching is made to the standby path on the receiving ends to recover from a communication fault.
Meanwhile, in the “1:1” method and the “Shared” method, as with the “1+1” method, a standby path having a path not overlapping with an active path is set when the active path is set. These methods are different from the “1+1” method in that resources for the standby path are reserved but switches are not set when the standby path is set.
Hence, in the “1:1” method and the “Shared” method, when an active path does not have a fault, resources reserved for a standby path can be used by extra traffic to effectively use resources in a network. Here, the extra traffic indicates communicated traffic which is supposed to be removed when the active path has a fault.
In the “1:1” method and the “Shared” method, when an active path has a fault, a signaling message is used to secure resources of a standby path and to set switches. When setting of the standby path is completed, it is possible to recover from a communication fault by making switching from the active path to the standby path.
An example of specific steps of the above-described fault recovery method is described in “draft-bala-protection-restoration-signaling-00.txt” (hereinafter, referred to as Document 2), which is an Internet draft of IETF. According to Document 2, in the “1:1” method and the “Shared” method, in the event of a fault on an active path, a start point node transmits a switch changeover request message which is transferred hop by hop to nodes on the route of a standby path until an endpoint node. The node having received the message secures resources for the standby path and sets switches.
When the message reaches the end point node, the end point node transmits a confirmation message which is transferred hop by hop to the nodes on the route of the standby path until the start point node. The nodes having received the message confirm whether the switches are set properly. When the message reaches the start point node, the fault recovery is completed. Here, the resources indicate an OXC port requested for setting an optical path, a time slot in SONET/SDH, and so on.
In the fault recovery steps of the “1:1” method and the “Shared” method described in Document 2, when the resources of a standby path are used by extra traffic, an improper connection occurs in such a way that data of a path which is under recovering from a fault is transferred to a client of the extra traffic.
This is because when the switches of the standby path are set from the upstream side of the path and data transfer is started, the switches of nodes on the downstream side of the path are set on the route of extra traffic, and thus data is supplied along the route of the extra traffic.
Further, in the fault recovery method of the “1:1” method and the “Shared” method described in Document 2, when a plurality of paths is set in a fault section, some of the paths may not satisfy a requested fault recovery time.
This is because the order of fault recovery is not managed on each node and thus a fault recovery operation may be started from a path not requiring quick fault recovery, thereby delaying the start of a fault recovery operation for setting fault recovery of the other paths requiring quick fault recovery.
Moreover, in the fault recovery steps of the “1:1” method and the “Shared” method described in Document 2, when a plurality of faults occurs simultaneously, fault recovery may not be performed, resulting in insufficient reliability.
This is because failed setting may occur on the standby path due to multiple faults resulted from the standby path as well as the active path, failed transfer of a signaling message for setting the standby path, and so on.
Furthermore, Document 2 does not describe the step of switching-back from a standby path to an active path when the active path recovers from a fault.